On Desktop, when you choose Group By -> Person, it shows you groups for every entity of that type you have added, plus an extra group at the bottom called "No Person". That shows any events that are not yet allocated to any of the groups above. On iOS, you can only view one group at a time. That "Unallocated XXX" item is the option to choose to show the "No XXX" group - i.e. Showing events that are not allocated to any other Person. This is most useful in project planning, when you need to see what work has not been allocated yet. In fiction, it may also highlight events that do not have complete information - I.e. You have added the event but not yet set its Participants etc.